Utilization Patterns of Multidetector Computed Tomography in Elective and Emergency Conditions: Indications, Exposure Risk, and Diagnostic Gain

Computed tomography (CT) has now become a first line of imaging modality for radiologists in the diagnostic workup of patients. Multidetector (MD) CT technology allows rapid examination and dynamic study after intravenous contrast medium administration; wide-ranging information regarding the status of the vessels as well as of the parenchymal organs can be obtained from image analysis. In this electronic era, postprocessing at a dedicated workstation allows us to make use of image reformations in a simple and direct way. From detection of a small nodule in the lung parenchyma or a small polyp of the colon to an active bleeding source or tiny lesion of the trachea, the “real” and the “virtual” but evident findings represent the daily routine for radiologists. In this background, the destiny (in some institutions probably the actual rule?) of the MDCT seems to be what in the past the conventional plain radiography was: a relatively “simple and rapid” method to make a clinical diagnosis. When the “easy and fast” becomes the “standard,” it seems that the risks of overuse are often ignored. As attested by Brenner and Hall in 2007,1 the increasing exposure to radiation in the population could represent a public health issue in the future, although the risk for any 1 person is not large. The critical point related to CT is related to the “dose”: the absorbed dose represents the energy absorbed per unit of mass (measured in Grays); the organ dose majorly determines the level of risk to organ from radiation; and the effective dose (measured in Sievert) is proportional to a generic estimate the overall damage to the patient from radiation exposure.1 The organ dose is the preferred quantity to estimate the radiation risk.1 Radiation doses to organs from CT examination depends on several factors1 such as the number of scans, the tube current and scanning time, the size of patient, the axial scan range, the scan pitch, the tube voltage, and the type of scanner used.1, 2 Advances in MDCT technology have made isotropic data acquisition feasible for nearly every application,3 and the benefits of routine use of isotropic data for image display and interpretation are known.3 However, the parameters that affect the radiation dose vary considerably in accordance with the CT scanner design, and those variations determine the cost in dose increase relative to the voxel size.3 Image acquisition using narrow or wide detector configuration and beam collimation can also affect the voxel size and the relationship between spatial resolution and the radiation dose.3 Image quality is proportional to radiation dose,4 and improvement in image quality implies an increased radiation dose.4 Comparison of the quality of MDCT images obtained with different detector configurations on scanners with 4, 16, 40, and 64 channels and the estimated radiation exposure incurred with each option, may allow radiologists to better understand the relationship between radiation dose and voxel size,3 to help balance the need for diagnostic image quality against the concern for patient safety.3

The problem of safety in radiation dose related to CT examination has been considered in various studies and observations. With the new MDCT technology and the introduction of hybrid systems, the volume of diagnostic procedures involving the use of ionizing radiation continue to increase, including in the pediatric population as well as screening procedures in asymptomatic adults.5 In an article discussing Alliance for Radiation Safety in Pediatric Imaging Vendor Summit, stakeholders were invited to discuss the development of better estimates of pediatric patient radiation dose.6 In fact, medical physicists currently use 2 standardized phantoms to estimate CT patient radiation dose;6 however, the adult model may underestimate displayed pediatric CT radiation dose on the console of current CT machines.6 These stakeholders agreed to partner to improve CT radiation dose estimates for children.6 The induced potential iatrogenic malignancy from diagnostic radiation is considered a crucial concern: according to major national and international organizations responsible for evaluating radiation risks, it seems that there is no low-radiation threshold for inducing cancer5 and, consequently, no amount of radiation should be considered “safe.”5 To reduce the overall radiation dose from CT procedures in the population, it is important to adjust scanner parameters separately for each individual, to keep radiation dose as low as possible.5

In a study aimed to assess patient dose and occupational dose in established and new applications of MDCT fluoroscopy, the study revealed high effective patient doses mainly for relatively new applications such as CT fluoroscopy-guided radiofrequency ablations using MDCT, vertebroplasty, and percutaneous ethanol injections of tumors.7 Beyond complex procedures and expected benefits of the treatments, what should be considered seems to be the general health state of the patient to justify any observed high effective patient doses.7

Indications for a CT examination, particularly in young and/or asymptomatic patients should be carefully considered.5 A critical point to consider is that the appropriateness of a diagnostic imaging tool must be kept in radiologist's hand. The referring clinician may have to be educated by the radiologist as to the most appropriate modality for the clinical question. Comprehension and knowledge of the need for imaging the patient should be a team approach. When referring clinicians or surgeons seek a radiologist's opinion, the radiologist must be well informed to provide precise answers, particularly in the context of justification of a modality that delivers low-dose ionizing radiation. With this aim, for example, a US national campaign in radiology designed to promote the need and the opportunities to decrease radiation to children when CT examination is indicated, demonstrated that simple and direct safety messages on radiation protection targeted to medical professionals throughout the radiology community, using multiple media, seem to be able to affect awareness potentially leading to change in practice.8

As attested in a 2008 publication,5 the risk of radiation-induced cancer is much smaller than the risk of cancer from natural sources; however, it can become a public health concern if large numbers of the population undergo increased numbers of CT screening procedures that may even be of uncertain benefit.5 In this order, CT use needs to be evaluated on a case-by-case basis,5 considering also the use of low dose and to account the real short-term benefit for the patient. Recently, virtual colonoscopy and cardiac CT angiography is being used as diagnostic or screening tool by physicians and patients. Due to the increased number of these examinations, the use of low dose and further improvement in new technology CT scanners to reduce radiation dose should be aimed for without loss in diagnostic effectiveness.9, 10

Frequent exposure to possibly carcinogenic levels of ionizing radiation from helical CT scanning is considered a potentially large public health issue for the medical community.11 Many studies have shown that organ doses associated with routine diagnostic CT scans are similar to the low-dose range of radiation received by atomic-bomb survivors.12 The food and drug administration estimates that a CT examination with an effective dose of 10 mSv may be associated with an increased chance of developing fatal cancer for approximately 1 patient in 2000,12 whereas the biological effects of ionizing radiation VII lifetime risk model predicts that with the same low-dose radiation, approximately 1 in 1000 individuals will develop cancer.12 The general attention to problems related to radiations is high;13, 14 in April 2007, the American College of Radiology released the “White Paper on Radiation Dose in Medicine”15 in which the panel concluded that the expanding use of imaging modalities using ionizing radiations such as CT and nuclear medicine may result in an increased incidence of radiation-related cancer in the exposed population. As potential solution to this problem, a prevention of the inappropriate use of such imaging modalities and a studies optimization was proposed to obtain the best image quality with the lowest radiation dose.14 Additional practical suggestions to minimize radiation risk were also to include education for all stakeholders in the principles of radiation safety and preferential use of alternative (nonionizing) imaging techniques (magnetic resonance imaging and ultrasound).15

The use of alternative diagnostic modalities to MDCT could be potentially easier to adopt in elective scenarios, whereas in an emergency setting, the need for precise information related to imaging findings in as short a time as possible makes alternative imaging methods less desirable, particularly in the critical patient. In traumatized patients, conventional radiography and ultrasound examinations can represent the basic diagnostic tools;16 however, MDCT with its shorter scan time and increased accuracy has become the gold standard for many indications in trauma imaging.16 Because of the higher radiation dose, the use of MDCT should be carefully assessed in younger patient population,16, 17, 18 and optimization of imaging parameters has to be performed to minimize exposure and maximize diagnostic safety.16 Beyond these undoubted concerns and required attentions, trauma care benefits from the use of imaging technologies.19 However, questions and problems related to the use of the MDCT examination in trauma still exist: CT scans should not replace careful clinical examination and should be used only in appropriate patients;19 by contrast, potential risk from missed important and often unsuspected findings that could be evident at CT examination must be an additional consideration to be kept in mind.

A recent study estimated cumulative radiation exposure and lifetime attributable risk (LAR) of radiation-induced cancer from CT scanning of adult patients at a tertiary care academic medical center.20 The study comprised 31,462 patients who underwent diagnostic CT in 2007 and had undergone 190,712 CT examinations over the previous 22 years.20 Thirty-three percent of patients underwent 5 or more lifetime CT examinations, and 5% underwent between 22 and 132 examinations.20 Fifteen percent received estimated cumulative effective doses of more than 100 mSv, and 4% received between 250 and 1375 mSv.20 CT exposures were estimated to produce 0.7% of total expected baseline cancer incidence and 1% of total cancer mortality.20 Seven percent of the patient population had estimated LAR greater than 1%, of which 40% had either no malignancy history or a cancer history without evidence of residual disease.20 Cumulative CT radiation exposure added incrementally to baseline cancer risk in the cohort:21 most patients accrue low radiation-induced cancer risks, but a subgroup seems to be potentially at higher risk due to recurrent CT imaging.20

The widespread use of MDCT in emergency seems to be gaining interest all round the world. Another recent study evaluated the justification and radiation risk of patients who undergo multiple CT scans during their hospital stay for emergency condition,22 where Griffey and Sodickson23 (2009) defined a conservative estimate of the number of patients undergoing repeat or multiple emergency department CT studies, quantifying their cumulative CT radiation doses and LAR of developing cancer. In this study, all patients at a tertiary care adult academic medical center with at least 3 emergency department visits within a 1-year period that included CT of the neck, chest, abdomen, or pelvis were considered,23 identifying all diagnostic CT studies over the previous 7.7 years.23 The authors calculated cumulative radiation doses by summing typical effective doses of the anatomic regions scanned, and LAR using the population-averaged dose-to-risk conversion factor of 1 cancer per 1000 patients receiving a 10 mSv dose, in accordance with the seventh biological effects of ionizing radiation VII report.23 Repeat imaging of the same study type represented at least half of the imaging for 72% of the patient population and all the imaging for 12%.23 A small proportion (1.9%) of emergency department patients undergoing CT of the neck, chest, abdomen, or pelvis have high cumulative rates of multiple or repeat imaging24; this patient subgroup was considered to have a heightened risk of developing cancer from cumulative CT radiation exposure.23 In an Emergency situation the health benefit of CT-related diagnostic information is immediate24 and not rarely life-saving, whereas the risk of induced cancer is decades away,24 with conservative estimates of the benefit-to-risk ratio for CT equal to 100:1 and higher.24 As assessed by Boone24 (2006) that CT examination should not be performed for inappropriate indications.

In a recently published article by Huber-Wagner et al21 in 2009, a retrospective multicenter study was made to compare the probability of survival in patients with blunt trauma who did or did not undergo a whole-body CT examination during resuscitation. The authors found that 32% of 4621 patients who were considered underwent whole-body CT scan and the relative reduction in mortality based on trauma and injury severity score was 25%, whereas that based on revised injury severity classification was 13%.21 This result seems to attest that whole-body CT examination in polytrauma patient can reduce the probability of death.21 Although the greater radiation exposure of a whole-body CT examination and the potential dangerous effects, the better diagnostic accuracy has to be considered: the early recognition of findings implies a prompt therapeutic plan.21

The usefulness of a prompt diagnosis and the risk from radiation due to CT examination have to be evaluated case-by-case, particularly but not exclusively, in emergency, where the time has a very high cost in terms of life-saving for patients care and treatment.